For quite some time, motion tracking arrangements for the measurement and tracking of the eye movement (eye trackers) have been utilized in ophthalmology and other fields of application for, among others, the purpose of research, diagnosis, and therapy. For example, during a treatment of the eye (cornea, retina) by means of a surgical laser, it has to be ensured that the input of light energy takes place de facto in those areas in the eye, which are to be planned in advance of the treatment, in order to avoid damage to the eye. This is achieved either through deactivation of the laser the moment an eye movement is detected, or through adjusting of the laser beam by means of illumination optics in accordance with the measured eye movement. Further applications of the eye movement measurement are the controls of machines, e.g., computers or motor vehicles.
According to prior art, the pupil of the eye is in most cases imaged repeatedly in succession by means of a high-speed video camera at a high image refresh rate, e.g., in accordance with WO 01/89438 A2. By means of specific image evaluation algorithms, e.g., through edge detection, the relatively sharp boundary between the black eye pupil and the lighter iris can be identified in the recorded images. In principle, the eye movement can be determined from shifts of said boundary and/or the pupil center in sequential images. In order to detect eye movements with smaller amplitude, the images must be taken with high optical resolution and sharpness. In order to also detect eye movements with large amplitude, the potential motion field of the pupil must be included in the images as extensively as possible. The combination of both requirements necessitates the detection of images with a high number of pixels.
However, the readout of such large images from the camera sensor and the subsequent analysis of, at least, partial image areas take a relatively long time due to the amount of data, resulting only in low, effective image refresh rates, i.e., rates for the recording, including the subsequent analysis, of no more than a few 100 Hz. However, the human eye can perform so-called saccade movements at a speed of approximately up to 600° per second. In order to achieve great accuracy of the motion measurement at such speeds, a significantly higher effective image refresh rate must be achieved. For the tracking-adjustment of a therapy laser during a femtosecond Lasik surgery, e.g., an effective image refresh rate of at least 1 kHz would be required. However, according to prior art, this can only be achieved through limiting of the spatial resolution or the observed maximum motion field.
The measurement of the movement of the eye fundus, particularly during a treatment of the retina, e.g., a laser coagulation, poses the additional problem that the necessary illumination must only lie within the infrared (IR) wavelength range in order to avoid a glare and therefore a blink reflex, even though the eye fundus is poor in contrast in the IR range. For example, IR illumination is used with non-mydriatic fundus cameras during alignment and adjustment. Visual light is only briefly introduced during a color image acquisition, which typically leads to a blink only at the end of the exposure time. In order to determine a movement of the eye fundus, particularly for the tracking-adjustment of a laser, with sufficient accuracy, the recorded IR images must be analyzed in significantly larger partial image areas than is required for edge detection on the pupil. As a result, the effective image refresh rate is further reduced. Alternatively, the movement of the eye fundus can be determined from a positional change of larger blood vessels. However, since the eye fundus only exhibits a few larger blood vessels, only a small portion of the eye fundus can be utilized for this type of analysis, hence measurements can only be taken in a limited motion field.
In addition to the motion measurement with the help of video recordings, the movements, particularly of the eye fundus, can be measured with scanning laser motion tracking arrangements. Said systems, known, e.g., from US 2006/228011, U.S. Pat. Nos. 6,726,325, and 5,644,642, confocally scan a preferably greatly structured portion of the eye fundus with a laser beam. Thereby, the scan can be performed circularly around a thicker vein branch or around the papilla of the eye. Said systems are advantageous because the sensors can be adjusted to the geometry of the specific eye. As a result, they only have to scan a few points of the eye fundus in order to detect movements with great accuracy, therefore allowing for a quick and uncomplicated evaluation of the image content. However, they are disadvantageous because an elaborate laser scanner is required.